[Big-Daddy]

When ICl reacts with the C=C group on a large molecule, I'm not sure whether I or Cl acts as the electrophile and nucleophile respectively. I've heard I is the stronger nucleophile, but on the other hand Cl is more electronegative than I so I would expect Cl to take the electrons when the bond breaks and thus I would act as the electrophile. Any help?

[discodermolide]

I think you are correct in your assumptions.

[Big-Daddy]

I think you are correct in your assumptions.

So in this case, I acts as the electrophile and Cl as the nucleophile (even though I is the stronger nucleophile)? But if you had a Cl substituent on a C atom, I would replace it because I is a stronger nucleophile?

[discodermolide]

Sorry I mis-read your original post. The Cl is the electrophile the I the nucleophile.

[Big-Daddy]

Sorry I mis-read your original post. The Cl is the electrophile the I the nucleophile.

I see. And in a case where you have a Cl substituted on a C atom in an organic molecule, an I would replace it (being a stronger nucleophile), but not the other way around?

How come the I-Cl bond breaks and I takes the electrons, if Cl is more electronegative than I?

[Dan]

The Cl is the electrophile the I the nucleophile.

I don't think this is right. The alkene nucleophile attacks the positively polarised (electrophilic) end of the interhalogen (iodine), expelling chloride. You then have an electrophilic iodonium that reacts with the nucleophilic chloride.

[souro10]

Sorry I mis-read your original post. The Cl is the electrophile the I the nucleophile.

Incorrect. Cl- being the more electro-negative atom will have a partial negative charge and Iodine will have a partial positive charge. Pi electrons of alkene will attack the partially positively charged Iodine atom, to produce the more stable carbocation/partial positive charge on one of the carbons if you're following the more correct iodinium ion mechanism, where the Cl- will attach.

" Iodine is a stronger nucleophile " - this statement may be correct under one set of conditions, incorrect under a different state of conditions. Example- in a polar protic solvent strength of nucleophile is I- > Br->Cl- > F- . while the reverse order exists in aprotic solvent.Reaction conditions - solvent, phase, substrate matter.

[Big-Daddy]

The Cl is the electrophile the I the nucleophile.

I don't think this is right. The alkene nucleophile attacks the positively polarised (electrophilic) end of the interhalogen (iodine), expelling chloride. You then have an electrophilic iodonium that reacts with the nucleophilic chloride.

Thanks.

So whichever is the less electronegative of the two atoms in the bond A1-A2 will be the electrophile, and whichever is the more electronegative will be the nucleophile?

And if this took place in the presence of H2O/OH-, the electrophile (I in this case) would go ahead and bind on to the less substituted carbon (in accordance with Markovnikov's rule), whereas the Cl- would simply dissociate off whilst the OH- bound on to the more substituted carbon?

Lastly, if you had a "weaker nucleophile" Cl substituted onto a C atom (i.e. a halogenoalkane group) and a "stronger nucleophile", e.g. I, came into contact with the mixture, am I right to assume that I replaces Cl in the organic molecule?

[souro10]

Yes, when interhalogens are being added to alkenes, the more electronegative of the halogens is the nucleophile and the less electronegative of the halogens is the electrophile, which attacks first.

Second question- It really depends upon the concentration of water or the OH-. If you do the reaction with water as the solvent, like the addition of bromine-water to alkenes, then water would add at the more substituted carbon , not because water is a stronger nucleophile ( water is in fact a weaker nucleophile than Cl- ), but because the number of water molecules far out-numbers the number of Cl- molecules trying to attack.

Last question- It doesn't always mean that if I- is present it'll replace Cl- . There must be favorable conditions - depends a lot on the substrate and the solvent.

You may be interested in Finkelstein reaction, the reaction rates in different halides.

Hope that helps.

[Big-Daddy]

Yes, when interhalogens are being added to alkenes, the more electronegative of the halogens is the nucleophile and the less electronegative of the halogens is the electrophile, which attacks first.

Second question- It really depends upon the concentration of water or the OH-. If you do the reaction with water as the solvent, like the addition of bromine-water to alkenes, then water would add at the more substituted carbon , not because water is a stronger nucleophile ( water is in fact a weaker nucleophile than Cl- ), but because the number of water molecules far out-numbers the number of Cl- molecules trying to attack.

Last question- It doesn't always mean that if I- is present it'll replace Cl- . There must be favorable conditions - depends a lot on the substrate and the solvent.

You may be interested in Finkelstein reaction, the reaction rates in different halides.

Hope that helps.

Thanks, with regards to the first two questions.

For the last one: in general we take stronger nucleophiles as replacing weaker ones, no? e.g. NH2^- or OH^- will readily replace any halogen in most solutions. By that reckoning I should replace Cl as substituent ...

(My reasoning is that a solvent will become protic if there is any halogen bonded to it, after which I is the most nucleophilic halogen and should therefore replace the rest. The exception is of course CCl4, since the dipoles cancel, in which case only F should be able to replace the Cl substituents. I know these are generalizations but I'm looking for an overall picture. Are these fairly accurate?)

[souro10]

Yes, when interhalogens are being added to alkenes, the more electronegative of the halogens is the nucleophile and the less electronegative of the halogens is the electrophile, which attacks first.

Second question- It really depends upon the concentration of water or the OH-. If you do the reaction with water as the solvent, like the addition of bromine-water to alkenes, then water would add at the more substituted carbon , not because water is a stronger nucleophile ( water is in fact a weaker nucleophile than Cl- ), but because the number of water molecules far out-numbers the number of Cl- molecules trying to attack.

Last question- It doesn't always mean that if I- is present it'll replace Cl- . There must be favorable conditions - depends a lot on the substrate and the solvent.

You may be interested in Finkelstein reaction, the reaction rates in different halides.

Hope that helps.

Thanks, with regards to the first two questions.

For the last one: in general we take stronger nucleophiles as replacing weaker ones, no? e.g. NH2^- or OH^- will readily replace any halogen in most solutions. By that reckoning I should replace Cl as substituent ...

(My reasoning is that a solvent will become protic if there is any halogen bonded to it, after which I is the most nucleophilic halogen and should therefore replace the rest. The exception is of course CCl4, since the dipoles cancel, in which case only F should be able to replace the Cl substituents. I know these are generalizations but I'm looking for an overall picture. Are these fairly accurate?)

Well, some of the factors you mention are correct, but there are always other factors.
For example if you use alcoholic KOH in any secondary or tertiary alkyl halide, you'll get the elimination product as the major product, and not the substitution product.

Another thing, NH₂^- is highly basic. In most cases, its basic nature is more dominant than nucleophilic nature, and so it prefers to go for elimination. Indeed, even for dehydrohalogenation of vinylic halides, we use NaNH2 in liquid ammonia - such strong is its basic nature.

If you use NaI in acetone, you will get the Cl or Br of an alkyl halide replaced by I , in most cases.
But there are peculiarities in this reaction , too. It is observed that reaction rate is high for benzylic , primary position , but reaction rate (and hence yield - the faster the product formed, the more of it will be there in the reaction mixture ) is slow for secondary and even tertiary position. 
But it'd be wrong to assume that I- will replace other halogens attached to organic compound in most solutions.

So you see generalizations like this cannot be done very easily. It depends on many factors- learn to see the factors and predict the product on a case-to-case basis. All we know for certain for alkyl halides is- halogens are very good leaving groups and participate in substitution reactions- but there may be competition with elimination reactions. Everything else depends on the specific reaction conditions- and any generalized statement will be wrong- because there will be numerous exceptions to it! Often, the prediction will be wrong. Because we do not know the whole of Organic Chemistry yet, many reaction mechanisms are unexplained. Remember that its an experimental science, we observe products from experiments and use the result and our theory to predict things under similar conditions. But unexpected products may arise- we are often oblivious to many things!